Composite paperboard containers for packaging foodstuffs and various other commodities constitute commercially significant products. These containers are well known to consumers as packaging for various foodstuffs including frozen drinks, ready to eat snack products, powdered beverages, nuts, dough products, and various non-food products including adhesives, sealants, etc. Typically, the containers are formed of a single ply or multiple ply paperboard body wall; an interior liner formed of one or more layers to protect the contents of the container from moisture and/or oxygen; and an exterior label layer identifying the contents of the container. End cap or closure members are attached to one or both ends of the container body and are typically formed of metal, plastics, films or film laminates, or composite paperboard materials. In some cases, one end of the container can be formed of a peelable film layer covered by a protective cap.
In many cases the container bodies have a circular or round cross-section, see for example, U.S. Pat. No. 3,892,351 to Johnson et al. However the container bodies can alternatively have a non-round cross-section, see for example U.S. Pat. No. Des. 382,446 to Bacon.
The composite container bodies are produced by various processes including the spiral winding, convolute winding, and linear draw processes which are conducted by wrapping or winding a structural paperboard body wall ply or plies, and a liner ply and/or label ply around a stationary mandrel to form a continuous tubular body. The tubular body is cut into segments, each having the desired length for a single container. Subsequently, a closure member is applied to one end of each container body to thereby provide an open container that can be filled with the desired product. After the container is filled with product, the second closure member is applied to seal the open end of the container.
The process of applying the closure members to the container ends typically involves the application of considerable axial force to the container body. In some cases, a rolled lip is formed on one end of the container body in order to provide a sealable surface for a peelable film layer and/or to support a snap-on overcap. The operation to form such a rolled lip similarly involves the application of considerable axial load to the container body. In addition, the filled containers are often subjected to considerable axial load during shipping and storage because multiple layers of packaged product may be stacked on top of each other during shipping and storage and/or multiple cartons of the filled containers may be stacked on top of each other or provide support for cartons of other products.
Because of such axially applied forces that the composite paperboard container bodies must withstand during manufacture, transportation and storage, the strength and thickness of the single ply or multiple ply paperboard material forming the body wall is selected to provide the necessary axial crush strength. This is typically achieved by using relatively high compression strength paperboard to form the single or multiple body wall plies and/or by selecting the thickness of the ply or plies to provide a specific body wall thickness calculated to meet structural requirements. Typically, the paperboard materials have a normalized compression strength, i.e, strength per unit area measured in the machine direction according to the standard "STFI" TAPPI procedure, of greater than about 1800 psi (pounds per square inch, 1 psi=6895Pa, 1Pa=N/m.sup.2) and typically the paperboard portion of the body wall has a thickness greater than about 0.014 inch.
Recently, different improved constructions of multiple layer paperboard winding cores for wound products such as films, yarns, and the like, have been disclosed. The different constructions each provide specific enhanced strength properties, tailored for the end use of the winding core. For example, U.S. Pat. No. 5,505,395 to Qiu et al. discloses spirally wound paperboard winding cores having enhanced resistance to inside diameter reduction resulting from the radially applied pressure of a tightly wrapped film or yarn material. In these constructions, the cylindrical body wall of the winding core is formed from three or more structural paperboard layers. The middle paperboard layer or layers used to form the body wall is formed of a low strength, low density paperboard material while the paperboard layers forming the outside and inside portions of the winding core are formed of higher strength, high density paperboard materials. On the other hand, U.S. Pat. No. 5,393,582 to Wang et al. discloses that paperboard tubes of enhanced flat crush strength may be formed from three or more plies of paperboard materials arranged to provide high compression strength paperboard materials as the middle or central portion of the body wall and low density, low strength paperboard materials at the outside and interior portions of the walls of the paperboard tube.
In winding core constructions, such as those discussed above, the axial strength of the tubes is not generally a design parameter because winding core constructions normally provide an axial strength exceeding the requirements imposed by that end use. In general, this results from a relatively high wall thickness, the use of many paperboard plies, and/or from the relatively high strength of the paperboard materials used to form winding cores.
Although the strength properties of composite paperboard container bodies can likewise be enhanced by forming the paperboard bodies of many paperboard plies, by using high strength paperboard materials, and/or by increasing body wall thickness, these modifications significantly increase the cost of the container bodies particularly in view of the liner and label plies that must be applied to the single ply or multiple ply paperboard body during the container body forming process. Accordingly, as a practical matter, the paperboard body construction techniques to enhance axial crush strength, currently available to those skilled in the art, involve use of paperboards of increased strength, and/or increasing body wall thickness, and/or the choice of one, two, or three paperboard plies as the body wall materials.